Collections - Exception

[Alexsandra Carvalho]

Hi,

Why is there a java.lang.UnsupportedOperationException at the line 3??

String[] s = {"1","2"}; 
List list = Arrays.asList(s);
list.add("3"); //3

[Bert Bates]

Hey Alexsandra,

From the "teach a person to fish" department:

Did you read the API concerning this method? It is a little complicated, but for sure that's a great place to start! No matter what other study materials you're using, it's a good idea to get comfortable reading and using the API doc.

So, it would be great to see a question like:

"The API says that this method, blah, blah, blah and I tried X,Y, Z, so I'm confused about ABC"

hth,

Bert

[Alexsandra Carvalho]

Hi,

I read the API and I think the problem is that the asList() method return a List with a fixed-size, and, therefore, I can't add or remove any element.

Is that?

[Roberto Perillo]

Howdy.

The problem is that this method "links" the list to the array. The size of the array is being set to 2 when you say {"1", "2"}. When you try to add something to the list that is linked to the actual array, you're trying to change the size of the array. You can't do that on the fly.

This code shows that the "asList(Object[] array)" method links the array to the list:

String [] s = {"1", "2"};
List<String> list = Arrays.asList(s);
s[0] = "Howdy!";
System.out.println(list.get(0));
 
Result: prints "Howdy!"

You're changing the array, and this reflects on the list.

[Alexsandra Carvalho]

Very good Roberto .
Thank you!!

[Bert Bates]

Good job Alexsandra!

[camilo lopes]

because the method asList return a String? this correct?

[Roberto Perillo]

Originally posted by camilo lopes:
because the method asList return a String? this correct?

Howdy.

No. The asList method returns a List. To be more specific, it returns an ArrayList, but since ArrayList implements List, then you can set the return to List (which is even better).

[Campbell Ritchie]

You are right the the asList() method returns an ArrayList, but it is actually a different class from the ArrayList we are familiar with. It just happens to have the same name.

[Roberto Perillo]

Now that you mentioned... I hadn't realized that the Arrays class has an internal static class called ArrayList (which also implements List). That's the one!